Purification of coal distillation gases



Jam 17, 1933.. A. POTT L$94A19 PURIFICATION OF COAL-DISTILLATION GASES Filed Jan. 7, 1929 Patented Jan. 17, 1933 -ITED' STATES ALFRED POTT, OF ESSEN-ON-THE-RUHR, GERMANY rwnrrioarron or COAL DIS'I'ILLATION easns Application filed January 7, 1929, Serial No.

The problem with which the invention is concerned, is to subject coal distillation gases, which have ordinarily been exposed beforehand to the usual purification, to a super-purification with the object of removing residues or traces of naphthalene from the gas, so that a gas, which according to present methods of investigation is free from naphthalene, is discharged into the adjoining conduits. The naphthalene content of coal distillation gases which discharge for instance into town mains at a low pressure or ifito long distance mains at a higher pressure, has always involved the greatest difficulty, since in the course of time it may cause sectional contraction or clogging of the mains. If the gas for any unavoidable reasons, should cool below the temperature at which it has been introduced under pressure into the conduit, and at which it contains naphthalene in va por form, it is impossible to prevent more or less extensive deposition of solid naphthalene, which adheres to'the walls of the conduit and at first forms incrustations, but in the course very readily, it was still impossible to produce a gas free from naphthalene; on the contrary it retained amounts of naphthalene which could not merely be determined by the usual methods of investigation, but were of such a magnitude as to make it likely that the undesired clogging of the passages would take place. Attempts were therefore made to prevent deposits in solid form at least, in the connecting pipes and the gas was carburetted,

washing oils for the naphthalene being therefore incorporated therein and the gas treated in this manner left for a time. If the gas for some reason became cool at any point,

either in the works or in the main leading to 50 the consumer, to such an extent as to cause-the fied if any cooling takes place, is conveyed.

of time leads to appreciable contraction or- 330,971, and in Germany January 14, 1928.

separation of solid naphthalene, the carburetting oil, which was chosen with a suitably low boiling point, was deposited at the same time, but in liquid form and brought the naphthalene down into solution, the whole being thenoonveyed to collectors of some kind and removed, If however, there did not happen to .be any cooling, the naphthalene vapor and carburetting medium were carried by the gas to the consumer. If one considersthe increasing amount of gas, which is produced today at gasworks and has to be conveyed by long-distance mains, it is evident that enormous quantities of the carburetting medium in some cases definitely escape with the gas, and in some cases necessitate a constant supervision and troublesome emptying of the collectors, to which the 'carburetting medium, containing naphthalene and lique- According to the invention the gas is treated with a solvent for naphthalene and permitted to absorb solvent vapors. This however 'is followed by a cooling of the gas to such an extent that the absorbed solvent vapors are condensed and at the same time remove the naphthalene completely from the gas- Consequently it is impossible by the methods of investigation in practice today, to find any naphthalene in the gas treated by the method forming the subject matter of the invention. The gas purified in this way cannot deposit any naphthalene either on its way through the works, or, if the purification has taken place at any other point, upon being conveyed from this place, even if there should by chance be any unavoidable and undesired cooling of the gas.

The solvent vapors are absorbed at temperatures, which are naturally higher than the cooling temperatures, subsequently employed, at which if possible the entire amount. of absorbed solvent vapors is to be liquefied again. It has been found especially advantageous to increase the temperatures, at which the gas absorbs the solvent vapors, above 40 and 50 C. to say, C, and thereupon effect the cooling. The minimum temperature of the latter must in its turn be such that a the solvent .vapors are condensed as completely as possible, to prevent the solvent which is usually expensive, from escaping.

with the gas into the pipes. In some circumstances it will therefore be advisable to choose the cooling temperature lower than is absolutely necessary or to subsequently lower the temperature employed for separating the solvent, inorder to eliminate any traces of the solvent from the gas and use the solvent for the same or other purposes. If in such. a case use is made of temperatures approaching or even below 0 C., the further result obtained is that the gas is completely dried. Gas has already been cooled considerably, even below 0 C. for removing the naphthalene from the gas. This has naturally nothing ,to do with the invention which does not employ either a washing with oil alone, or carburation alone or excessive cooling alone, which, experience has shown, would never attain the object of the invention. Similarly the invention has nothing to do with a method which aims exclusively at obtaining valuable materials, such as for instance gasoline, from a gas and bringing the latter into contact with the same washing oil first at an increased temperature and subsequently at a reduced temperature; by such a methodit is probably possible to remove the valuable material concerned from the gas, but not naphthalene, as a treatment of this kind would be equivalent to bringing the gas to be freed from naphthalene into contact exclusively with a washing oil of low temperature.

In practice the invention is carried out for instance in such a way that normally treated distillation gas of the usual temperature, with about 15 grms. naphthalene per 100 cbm. is allowed to bubble through a suitable solvent, for instance naphthalene oil, benzene washing oil, steam-volatile constituents of washing oil, tar oil etc. at temperatures of for instance 100 C. The most favorable temperature should however in each case be determined to suit the character of the washing oil. In such a case the liquid washing oil can become sufliciently enriched with naphthalene. In addition however the gas becomes charged with oil particles, which remain in vapor-form in the discharginggas. Upon subsequently passing through the cooling stage, these oil particles are deposited with simultaneous absorption of the naphthalene, which is still contained in the gas.

The inventors interpretation that during cooling, the-. 1st of washing oil vapors first formed, and """the subsequent condensing liquid in which the condensed particles of oil separate'from the gas, act so to speak with a very great surface development on the traces of naphthalene contained in the gas, and thus eliminate it completely in solution from the gas, which may explain the surprising result of the complete removal of naphthalene from the gas. Accordingly the solvent should be incorporated with the gas in a minimum quantity such as to ensure the complete washing out of the naphthalene.

The "solvent condensate containing. naphthalene is naturally separated from the gas, if necessary by reducing the temperature in stages. The solvent recovered in this way is now used for other purposes or, if desired freed more or less from absorbed naphthalene (regenerated) and/or mixed with fresh solvent, and supplied again to the initial stage of the process. If use is made of oils, which can be charged to a high degree with naphthalene, for instance up to 44%, it is possible to manage with comparatively small quantities of oil, which, when the whole process is suitably carried out and the oil condensate returned to the initial washing stage, require during the course of'time practically no ap preciable addition of fresh oil.

Any necessary regeneration of the solvent can be effected by cooling or driving off at least a portion of the absorbed naphthalene. A suitable apparatus for carrying out my improved process is illustrated in vertical section in the accompanying drawing, which shows a vessel a having an inlet 1 for receivmg the gas after it has been purified by the method heretofore used. This vessel is filled with a suitable washing oil up to or above the level of the inlet 1. The gas is discharged through the inlet 1 into the bottom part of the vessel beneath the surface of the washing oil. This gas is preferably under a pressure of 1 to 6 atmospheres while the temperature in the vessel is above 30 C. If the gas should be introduced under normal pressure, which is only a few millimeters as measured by a water column, then the temperature of the vessel would preferably be raised to or C. or higher. Coal distillation gas as it is sent over long distance is usually under a pressure of more than 1% atmospheres,

generally 5 to 10 atmospheres, and at such pressure will have a temperature about 100 C. Such gas may be passed through the vessel a without previously heating the latter, the oil in the vessel being heated directly by the gas. The gas, which has already been purified by the usual method heretofore used,

gives up while passing the oil in the vessel some parts of the remaining naphthalene and benzole which has not been removed by the previous treatment. Vapors of the washing oil are also taken up by the gas as it bubbles through the oil in the vessel. First the vapors of low boiling oil are thus taken up by the gas and flow therewith successivelythrough the cooling vessels 1; and c. Here the gas, together with the oil vapors, is cooled and the oil vapors are thereby condensed so that they form a sort of very fine rain of oil which goes down through the gas and takes out therefrom the last remaining traces of naphthalene. Such rain of oil develops a which the condensate is allowed to accumuthalene which they the oil and naphthalene due to the vessel a.

V by fresh'oil'.

ers can at the same time be used for conve large surface which comes in contact gas which is now discharged into the pipes. wit the oil and traces of naphthalene,thereimilarly however the organic solvent or by dissolving or absorbing it and taking it its vapors, with which the gas is allowed out of the gas. The gasa's it leaves the plant to be charged, either in the gas phase or durat the outlet 2 is completely devoid of any ing condensation in the cooling stage, can traces of naphthalene. e absorb in addition to traces of naphthalene The oil' condensed in the coolers b and 0 also traces of benzene and other impurities, flows thro'ugh the pipe 3 into a vessel d in which therefore, dissolved in the solvent, can

be condensed and removed with the latter. late for a suitable period of. time. Here the The substances which have been found par- Water which was taken out of the 'gas by the ticularly suitable for such a method are oil at the same time as the naphthalene is solvent'naphtha'II,"the,low-boiling steamcooled in the lower part of the vessel (1, while volatile parts of naphthalene oil or benzene their lower washing oil, more especially the coal tar oils specific gravity are cooled in the upper part boiling between 160220 C., which conseof the vessel. The water thus separated from quently have a considerably greater capacity the oil is withdrawn through the pipe 4' and of absorbing naphthalene at an increased the oil is withdrawn through the pipe 5 into temperature than at room temperature and f the vessel a where'it' again mingles with the therefore when charged and saturated with heavier oils which have remained therein. naphthalene etc. at an increased tem erature, Herein a part of the naphthalene contained deposit naphthalene during the su sequent in the incoming lighter oil is imparted to the cooling. If such oils are employed, the gas heavier oil because the latter has a capacity must be carefully cooled, to prevent any for dissolving naphthalene at a given temsolvent escaping with the purified gases and perature more readily than the oils which being Wasted. If the gas is kept under preshave a lower boiling point. The lighter oils sure during purification, the relative loss in are therefore partially freed from the naphsolvent, calculated on the volume of the gas have carried. If they under atmospheric pressure, will be extremeare now evaporated again by passing hot gas 1y small. Care will have to be taken however through them they are absolutely freed from that the gas leaves the cooling stage at temnaphthalene because only the washing oils peratures of 20 C. or less. A complete and not the naphthalene are evaporated. precipitation of the'solvent could be obtained If a mixture of heavier and lighter oils are by excessive cooling of the gas to 0, whereby treated in this manner, the lighter oils with an absolutely drying of the gas would also the lower boiling point apparently flow in a be obtained. Cooling to this extent requires closed circuit, first being evaporated in the however large and correspondingly expen vessel a, then flowing with the gas through sive coolers. the coolers and returning as condensates to Losses in washing oil and such low final.

,1 g V temperatures of 20 C. and less, can be avoidmixture in the vessel a has beed however by using higher-boiling oils than After the oil come aproximately saturated with naphthaordinary benzene washing oil, anthracene oil,

lene, the oil may be regenerated or replaced solvent naphtha II etc., i. e. higher or highboiling oils, such as for instance quickly dis- Although the invention is not limited to tilled benzene washing oil, quickly distilled any method of carrying out, and any suitanthracene oil etc. These high-boiling oils able solvent, that is to say more especially do not increase their absorption capacity for low and high-boiling oils alone or in miX- naphthalene, benzene, etc.' in the case of inture, may come into consideration, the invencreases in temperature, to the same extent tioncan be carried out with particular adas the low-boiling oils, i. e.those boiling up vantage and simplicity in cases in which gas to about 220 C. Consequently if a highis given off at a high pressure. as for instance boiling oil is saturated at room temperature in long-distance gas pipes. The gas is heated with naphthalene, its capacity of absorbing by the high compression and conveyed by a naphthalene is not increased by a rise in tem plunger into the solvent which it heats. In perature alone, to such an extent that it such a case the gas at an increased temperawould completely dissolve and remove ture (for instance C.) gives up naphtha-- naphthalene from the gas to be purified, at lene to the oil and becomes charged on the an increased temperature. In other words: other hand with oil vapors. Seeing however Purification of the gas with high-boiling oils that the gas which has become hot by comis notwithout further preliminary as simple .pression, must be cooled in order to change and complete as with low-boiling oils, If its volume, before it is discharged into the however naphthalene, benzene, etc. are ex-' long-distance pipes,-the corresponding coolpelled artificially from the high-boiling washing oil, as soon asthis washing oil is densing the washing oil vapors, while completely removing the naphthalene from the etc., and the oil regenerated in this ay thereupon re-introduced into the washing stage, it is possible completely to eliminate traces of naphthalene, benzene, etc. from the gas, without having to reduce the temperature of the gas .to be purified, in the cooling stage to about C. or lower.

On the contrary in that case a completely purified gas is obtained if it leaves the cooler at about -40 O. Losses in this washing oil in the discharging gas need not be feared nevertheless, even when working with only a slight or without pressure in the gas, because the' tension of such high-boiling washing oils is extremely small at room or moderately high temperature, i. e. about 30- 40 C.

Iftherefore use is made solely of higher or hi gh-boiling washing oils for carrying out the invention, the gas to be purified will again be allowed to bubble up through the washing oil at an increased temperature, in any case above 30 C. and preferably above 100 C. It will also be advantageous to allow the gas to become charged with vapors of this washing oil and thereupon condense these washing oil vapors in thecooling stage. The condensate will however in this case not be cooled simply to room temperature and again intro duced into the washing stage, but the absorbed naphthalene will be expelled therefrom, at least in part, by distillation; only then is this washing oil introduced as fresh oil into the washing stage. A further method of carrying out the invention is finally to employ an oil mixture for washing the gas, which mixture consists of highand low-boiling oils, i. e. preferably mainly of oils, which boil above 220 C. and the remainder of oils, which boil belowabout I "220?, hu

If the gas to be purified is a1loweddFlnilro1ighdrying otthe gasancttheeendensati on ble up (immersed washing) at an increased temperature above 30 (1., preferably above 100 (3., through an oil mixture of this kind, naphthalene, benzene, etc. are removed by the oil mixture from the gas. In addition the gas becomes charged with vapors con- I sisting chiefly of the low-boiling oils, which are now condensed in the cooling stage, form a fine precipitate and-at the same time dissolve any traces of naphthalene, benzene, etc. in the gas and remove them. The condensates, after separation from water, can be returned into the immersing stage, mix with the oils there, give up a portion of their naphthalene content to the high-boiling oils more especially and are thus made capable of passing again as solvent vapors into the gas to be purified, and of becoming enriched afresh with naphthalene, benzene residues etc. The low-boiling constituents of the oil mixture thus perform so to speak a circuit between the immersing and cooling stage, while the higher-boiling constituents of the a oil form a kind of naphthalene accumulator in the immersing stage, which is gradually increasingly enriched with naphthalene. The absorption capacity of the oil mixture for naphthalene is in such a case considerably higher than that of high-boiling oils alone, while on the other hand the vapor pressure of the mixture and at the same time the possible losses in oil are less than when lowboiling oils are used alone. A regeneration of the oil mixture is not necessary for some time. Consequently the naphthalene must not be crystal'lizedout after each condensation, before the washing oil is returned to the immersingstage, as is the case when highboiling oils are used.

When however the oil mixture is enriched with naphthalene to suchan extent that a complete purification of the gas is no longer obtainable, it is fed to the separator. In such a case the low-boiling oil is first distilled ofi, and the crystallized naphthalene expelled by centrifugal force from the residue in the still after cooling to room temperature. possible to obtain the high boiling constituents of the washing oil free from naphthalene again. These can be added again to the res- By quick distillation it is then idue from the light oil in any desired proportion, whereby a mixture of washing oil ready for use is again obtained.

When the water cooler 1s adjusted to such an extent that the purified gas flows off at temperatures of about 254 5 (3., a further intense cooling of the gas with freezing liquor of about 5 C. has proved advantageous. As

the gas in such a case is already completely free from naphthalene, this intense cooling has nothing to do with its purification from naphthalene. On the contrary this intense cooling is merely intended to effect a thorof any vapors of washing oil that may have been carried along. When the gas then leaves the super-cooler at a temperature of about 12 C. it is discharged completely free from- 'kinds of washing oil is used.

It is clear that the method forming the subject matter of the invention, however it may be carried out, is particularly economical for the purification of long-distance gas, 390211186 the latter owing to the necessary com pression is heated to a high temperature and the washing with oil thus takes place at this temperature, while on the other hand a cooling must also take place, and the last traces of naphthalene, etc., be washed out by the rain of washing oil condensate.

Moreover the invention is in no wav limrename ited to the absorption of naphthalene by the washing oil taking place in the immersion stage only, or super-purification taking place exclusively in the cooling stage by the condensed washing oil vapors. It is of no importance for the invention what processes occur in the immersion stage and in the cooling stage, whether for instance in the immersion stage the majority of the naphthalene is removed from the gas and a subsequent purification of the gas takes place in the cooling stage by means of the solvent carried along from the immersion stage, or whether no naphthalene is given off by the gas in the immersion stage, but the gas is only charged with solvent vapors and the whole purification takes place in the cooling stage, or whether finally at certain periods of the purification process when the oil is very highly charged With naphthalene, a portion of the naphthalene is carried from the oil in the immersion stage into the cooling stage, and returns, together with the condensate precipitated in the cooling stage, in a cycle into the immersion stage. On the contrary the chief point is that it is always possible completely to eliminate the naphthalene from a gas charged with naphthalene and that consequently the gas finally leaves the washing apparatus in a perfectly pure state.

In order to avoid losses in washing oil, itis possibleinstead of employing or in addition to employing intense cooling, to treat the gas with a high-boiling, oil absolutely free from naphthalene, which is prepared in a very simple manner by allowing the oil to be traversed at an increased temperature by air or gases free from oxygen, such as for instance flue gasesgcoke furnace gases. By this means all naphthalene is removed from the oil, without appreciable amounts of oil passing over into the gas. A high-boiling oil free from naphthalene and prepared in this way absorbs the constituents of the washing oil, boiling between 160-200 G. from the finely purified gas, if the latter is for instance sprayed with this oil, and the gas is therefore discharged into the pipes without carrying washing oil with it.

What I claim is:

1. The method of removing traces of naphthalene from coal distillation gas previously purified in the usual way, which comprises treating the gas in a first stage at a temperature above 40 C. with solvents for naphthalene, said solvents forming a mixture of light and heavy oils, thereby causing the incorporation of the vapors of said solvents in the gas, thereupon cooling the gas in a second stage, thereby causing simultaneously the condensation of said vapors of the solvent and the traces of naphthalene, this second stage belng performed before delivering the gas to the main, then delivering the condensed solvents back to the first stage for remixing with oils purified in the usual way, which comprises treating the gas in a first stage at a temperature above 40 C. with solvents for naphthalene said solvents forming a mixture of light and heavy oils, thereby causing the incor-.

poration of vapors of said solvents in the gas, then cooling the gas in a second stage and thereby causing simultaneously the condensation of said vapors of the solvent and the traces of naphthalene, then further cooling the gas in a third stage to complete condensation of the solvents the second and third stages being performed before delivering the gas to the main, and then delivering the condensed solvents back to the first stage for remixing with the oils and thereby increasing the temperature, thereby repeating the rocess.

3. The method of removing traces of naphthalene from coal distillation gas previously purified in the usual way, comprising the compression of the gas to a pressure adapted for long distance transmission, passing the gas so compressed and with raised temperature in a first stage through solvents for naphthalene, thereby causing the incorporation of vapors of said solvents in the gas, then in a second stage cooling the gas and thereby causing the condensation of the vapors of the solvents which also take up from the gas the traces of naphthalene, and then separating thiscondensate from the gas before delivering the gas to the main. I

In testimony whereof I have signed my name to this specification.

' ALFRED POTT. 

